1. Field of the Invention
One embodiment of the disclosed invention relates to a semiconductor device and a touch panel.
2. Description of the Related Art
In recent years, display devices such as e-book readers have been actively developed. In particular, a technique by which images are displayed using a display element with memory properties has been actively developed since it greatly contributes to a reduction in power consumption (Patent Document 1).
In addition, a display device provided with a touch sensor has attracted attention. The display device provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter simply referred to as a touch panel). Further, display devices provided with an optical touch sensor are disclosed in Patent Document 2 and Patent Document 3.
A light-emitting element using an organic compound as a light emitter, which has features such as thinness, lightweight, high-speed response, and direct current low voltage driving, has been considered to be applied to a next-generation flat panel display or a next-generation lighting device.
It is said that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with a layer containing an organic compound provided therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the layer containing an organic compound (also referred to as an EL layer) to form molecular excitons, and energy is released and light is emitted when the molecular excitons return to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.
The layer containing an organic compound has a layered structure typified by a layered structure of a hole-transport layer, a light-emitting layer, and an electron-transport layer. EL materials for forming EL layers are roughly classified into low molecular (monomer) materials and high molecular (polymer) materials. An evaporation apparatus is used for film formation of the low molecular material.
In addition, a light-emitting element including a cathode, an EL layer, and an anode is called an EL element. There are two types of EL elements: a type where an EL layer is formed between two kinds of stripe electrodes that are provided crosswise (simple matrix type); and a type where an EL layer is formed between pixel electrodes that are connected to TFTs and arranged in matrix and counter electrodes (active matrix type). However, when the pixel density is increased, the active matrix type where each pixel (or each dot) is provided with a switch is considered to be advantageous because it can be driven at a lower voltage.
In addition, a display device provided with a touch sensor has attracted attention. The display device provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter also referred to simply as a touch panel).
In a conventional touch panel, a pressure-sensitive sensor array or a capacitive sensor array is provided so as to overlap with a display panel; by touching a substrate of the sensor array with a finger or an input pen, the touched position is detected. Accordingly, three or more substrates are used in total because a pair of substrates is used for the display panel and the substrate of the sensor array is disposed so as to overlap with the pair of substrates.
Patent Document 4 discloses a structure in which a touch panel is provided on a display screen of an electroluminescence display device.
In order that the number of substrates overlapping with each other is two, the display device includes an EL element and a photodiode over the same substrate and includes an optical touch sensor; thus, the display device can be lightweight and thin. Patent Document 5 discloses a portable information terminal in which an EL element and a photodiode are provided in one pixel over the same substrate and which includes an optical touch sensor. Patent Document 6 discloses a display device in which a transistor including an oxide semiconductor and an EL element are provided in a pixel and which includes a photodiode.